Outputting a forms contents to a CSV file is incredibly handy and easy to do, this post will cover the steps involved.

Demo

First create the form, for this post it will collect a name and email address.

<form action='' method='post'>
<p><label>Name</label><br><input type='text' name='name' value=''></p> 
<p><label>Email</label><br><input type='text' name='email' value=''></p> 
<p><input type='submit' name='submit' value='Submit'></p> 
</form>

Next check if the form has been submitted.

if(isset($_POST['submit'])){

Then collect the data from the form.

$name = $_POST['name'];
$email = $_POST['email'];

Check if $name is empty if it is create an error array.

if($name ==''){
    $error[] = 'Name is required';
}

Check for a valid email address, if not valid create an error array.
 

if(!filter_var($email, FILTER_VALIDATE_EMAIL)){
     $error[] = 'Please enter a valid email address';
}

If no errors have been created then carry on

if(!isset($error)){

Next define the columns of the CSV file by comma separating the titles ending the row with a \n

$Content = "Name, Email\n";

Add the form data to the $Content, this will be the body of the CSV file.

$Content .= "$name, $email\n";

Next set the CSV filename and use headers to output the CSV file.

$FileName = "formdata-".date("d-m-y-h:i:s").".csv";
header('Content-Type: application/csv'); 
header('Content-Disposition: attachment; filename="' . $FileName . '"'); 
echo $Content;
exit();

If their are errors display them

if(isset($error)){
    foreach($error as $error){
        echo "<p style='color:#ff0000'>$error</p>";
    }
}

Final Code:

<?php
if(isset($_POST['submit'])){

    //collect form data
    $name = $_POST['name'];
    $email = $_POST['email'];

    //check name is set
    if($name ==''){
        $error[] = 'Name is required';
    }

    //check for a valid email address
    if(!filter_var($email, FILTER_VALIDATE_EMAIL)){
         $error[] = 'Please enter a valid email address';
    }

    //if no errors carry on
    if(!isset($error)){

        # Title of the CSV
        $Content = "Name, Email\n";

        //set the data of the CSV
        $Content .= "$name, $email\n";

        # set the file name and create CSV file
        $FileName = "formdata-".date("d-m-y-h:i:s").".csv";
        header('Content-Type: application/csv'); 
        header('Content-Disposition: attachment; filename="' . $FileName . '"'); 
        echo $Content;
        exit();
    }
}

//if their are errors display them
if(isset($error)){
    foreach($error as $error){
        echo "<p style='color:#ff0000'>$error</p>";
    }
}
?> 

<form action='' method='post'>
<p><label>Name</label><br><input type='text' name='name' value=''></p> 
<p><label>Email</label><br><input type='text' name='email' value=''></p> 
<p><input type='submit' name='submit' value='Submit'></p> 
</form>